Chain equivalence in reaction of nitric oxide with hemoglobin

Mixtures of nitric oxide and hemoglobin were prepared in a rapid freeze apparatus and analyzed by EPR spectroscopy. Spectra from samples at various degrees of saturation showed that the two subunits bound NO at equal rates. Identical results were observed in 0.1 M phosphate at pH 6.5 and 0.1 M 2,2'-bis(hydroxymethyl)-2,2',2''-nitrilotriethanol, 0.1 M NaCl at pH 7.0, both in the presence and absence of inositol hexaphosphate at either buffer condition. At subsaturating levels of NO (less than 60%), or at all levels of saturation in the presence of inositol hexaphosphate, it was found that the EPR spectrum of nitrosylhemoglobin varied with the length of time before freezing. This change was characterized by the development of a hyperfine structure at g = 2.01 which appeared with a half-time of approximately 0.4 s. Maxwell and Caughey (Maxwell, J. C., and Caughey, W. S. (1976) Biochemistry 15, 388-395) have attributed this three-line EPR hyperfine structure to the formation of a pentacoordinate ferroheme-NO complex. Corresponding slow changes were observed in the visible absorption spectrum following the binding of low levels of NO to deoxyhemoglobin or inositol hexaphosphate to fully saturated nitrosylhemoglobin. Thus it appears that NO binding to the alpha and beta subunits of deoxyhemoglobin takes place at equal rates and, under conditions favoring the T quaternary state (low saturation, presence of inositol hexaphosphate), a further slow structural change takes place, resulting in the cleavage of the iron--proximal histidine bond.